The answer is a highly subjective, according to both the wine and the drinker. Generally, dry white wines have the shortest life once opened.The

I find they lose their character after even one or two days, especially cheaply-made, oaky New World whites.

 
(Whites with good acidity and lots of character from Burgundy are an exception.)

For reds, I think that most start to slip after two days, though again it depends on how well the wine was made. There are always exceptions …

Sweet and fortified wines, such as icewine, port and sherry, have a longer life because of their higher sweetness and/or alcohol, both of which act as a preservative. I still like most opened icewines after three to four days; ports from one week to four, depending on their quality.

You can extend the life of any opened wine by giving it a few squirts of liquid nitrogen, such as Wine Preserver, a spray can sold in many liquor and wine accessory stores. Another trick is to pour your remaining wine into a clean, empty half bottle size and cork it.

This also minimizes the amount of oxygen that can affect the wine. Some drinkers don’t mind a wine that’s still 60-80% there in terms of its character; others want the full expression or nothing. I tend to err on the latter side of the ledger.

How long do you think wines keep once opened?